How to Efficiently Extend a Vector with its Own Content in C ?

Efficient Way to Extend Vector with its Own Content

In C , the vector container is widely used for dynamic memory management. When it comes to appending the contents of a vector to itself, without using a loop, developers often search for an efficient solution.

One approach involves using the std::copy function. However, as mentioned in the question, this can lead to a segmentation fault. The reason for this is that simply copying the elements to the end of the existing vector overlaps the memory occupied by the original elements.

A more reliable solution employs a combination of std::resize and std::copy_n. The following code snippet demonstrates this approach:

auto old_count = xx.size();
xx.resize(2 * old_count);
std::copy_n(xx.begin(), old_count, xx.begin() + old_count);

This approach works by first increasing the vector's capacity using std::resize. Since std::resize reallocates memory if necessary, we need to remember the original size using the old_count variable. The subsequent std::copy_n operation copies the original elements to the newly allocated memory, effectively duplicating the vector's contents.

An alternative to std::resize is std::reserve, which only allocates enough memory to hold the specified number of elements. However, after using std::reserve, std::copy_n is still required because the end iterator points one element past the end of the vector, making it invalid for insertion.

It's important to note that both std::insert and std::push_back may require reallocation, which can invalidate existing iterators and references before the insertion point. Therefore, for the purpose of appending a vector to itself without using a loop, the combination of std::resize or std::reserve and std::copy_n provides a reliable and efficient solution.

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